The hypothalamo-pituitary-adrenal (HPA) axis is the major stress axis in humans and other mammals. A variety of stressors (and multiple other classes of stimuli) cause release of the hormone ACTH (adrenocorticotropic hormone) from the anterior pituitary gland. ACTH enters the systemic circulation and acts on the adrenal cortex to promote synthesis and release of glucocorticoid hormone (the major endogenous glucocorticoid being cortisol in humans and corticosterone in rodents). The glucocorticoids exert a broad spectrum of effects, the main purpose of which is to mobilise energy sources for successful responsiveness and eventual adaptation to the stressor.